JeepXJ93 said:
Ohhh thats where i went wrong, magazine said "New tire diameter, over Old tire diameter" and i have 31's on it assuming that is my "old" tire diameter, ok so i came out to the same thing now, is a 4.56 really better for on the road gas mileage than the 4.10? Thanks again for clearing this up for me. 5-90 i'm looking into tires now, i have the trxus mt in a 31 and i'm bent on what to go w/ now in the 33, i want something good for (separately) rock, mud, and snow conditions. Any suggestions on that greatly appreciated.
"What gearing should I use for the street" has a lot to do with tyre size. Let me dig out my maths book...
OK, got it! Put on your thinking cap, and get out your notebooks...
First and foremost, remember that gear ratios are
multiplicative, not additive. This means you multiply transmission gear ratio by axle gear ratio to get an "overall" gear ratio. This is important - don't do it, and you're going to get wrong answers... Generally, it's easier to figure "cruise" RPM one gear short of overdrive, which should be 1.00:1. However, do
not guess - this is exact science. It would take me some digging, but I probably have gear ratios for most common transmissions, if you can't find them anywhere else (on the "someday list" for coding into HTML.)
The basic formula is:
RPM = (mph x overall gear ratio x 336)/(tyre diameter); with tyre diameter in inches. This can be re-arranged algebraically into:
Gear Ratio = (rpm x tyre diameter)/(mph x 336). Note that this is still
overall gear ratio - if you plan to cruise in fourth, you're good with something close to this number. If you plan to use overdrive, then you'll have to
divide this number by your overdrive gear ratio to select your axle ratio.
Typically, cruise speed can be taken as 65 or 70mph. And, select anything from 2600-3000rpm for cruising engine speed, with preference toward 2800rpm. So...
GR = (2800 x 30.8)/(65 x 336) (I'm using BFG 31's here...)
GR = 86240 / 21840
GR = 3.94 - so you can split the difference and go with either 3.73 (for primary street use) or 4.10 (if your focus is off-road.)
Granted, this assumes that you're going to cruise in fourth gear. Since (using another example) the Aisin AX-15 uses an overdrive of 0.79:1 (21%,) your gear ratio "core number" would end up being 3.94 / .79, or 4.98:1 (meaning either 4.88 or 5.13 gears to cruise in overdrive.) You may be better off not using fifth gear.
Since the AW4 uses an overdrive ratio of 0.75:1 (25%,) you'd want to change that to 3.94 / .75, or 5.25:1 (meaning 5.13 gearing.) If you have an AW4, start thinking about getting either a D44 or Ford 9" for the rear...
5-90